Package name is missing in XML report.

Since 3.6 the package name is missing. I think the problem is the optimization of the package name. See xmlreport.py line 95 (xml_file):
- The cu.name is a file path in my case (e.g., src\sample_calculator__init__)
- rparition goes wrong => creates an empty string because cu.name has no "."
I used coverage.py 3.6 on Windows 7 with Python 2.7.3